Damage to Aging Thymic Stroma and De Novo T Cell Development.

Successful T lymphopoiesis is essential for maintaining resistance to infection, mediating graft versus tumor activity, and as a platform for some immune based therapies. Among the most profound age-related change affecting the hemopoietic system is involution of the thymus and a decline in T cell production. Coordinated interaction of marrow-derived lymphoid progenitors with thymic stromal cells is required for successful de novo T lymphopoiesis. Current models of thymic involution indicate that aging alters the capacity of thymic stromal cells to support this de novo T cell lymphopoiesis. Loss of specific sets of thymic stromal cells, or loss of specific lympho-stromal signaling interactions may partially explain age-based impaired T cell reconstitution. Prior models examining the potential to reconstitute the aged thymus have been conducted either in vitro or in cyto-reduced hosts, and thus largely risk confounding cytotoxic damage with age-related damage. We have established an in vivo functional assessment of the ability of thymic stroma to support de novo T lymphocyte development. In the absence of radiation or chemotherapy this system supports robust thymic reconstitution after transplantation of limited numbers of T cell depleted wild type precursors. We have previously demonstrated that the capacity for reconstitution in this model system depends on the presence of normal, but empty, stromal niches. Using this model we have demonstrated that a variety of chemotherapeutic agents damage the thymic stroma and impair robust reconstitution. We now present data demonstrating that with increasing age reconstitution becomes less robust. Thus in mice older than 16 months unlike in those younger than twelve months, donor derived T lymphocytes can be identified, but the absolute cellularity of the thymus does not increase after transplant. In addition, we demonstrate that there are agent specific defects in the stromal compartments of treated aged mice. We have proceeded to identify a subset of agents (including split dose irradiation) that impair reconstitution in aged recipients but do not do so in the young. Thus while reconstitution of the thymus of young mice subjected to hyperfractionated total body irradiation declines 28 +/− 3% that of old mice treated in the same manner declines 91 +/− 6%. This last finding has enabled us to identify specific “at risk” stromal populations that normally appear to support the DN2 to DN3 transition in T lymphopoiesis. In a series of experiments designed to ask whether these at risk stromal populations can be protected from injury we identify agent specific differences in mechanisms of injury. It is anticipated that results from these investigations will augment our understanding of lympho-stromal interactions crucial to normal intra-thymic T cell development.